This invention relates to hollow, pipe-like or tubular ceramic bodies used as gas conducting channels and particularly to ceramic bodies used as exhaust ducts of a cylinder head of an internal combustion engine. More particularly, the present invention relates to a means of strengthening the ceramic bodies by the application of a pressure resistant coating or layer.
Ceramic bodies of this type are included in a cylinder head when the cylinder head is cast from a material such as an aluminum alloy. Relatively high structural loads are exerted on the ceramic bodies during the casting operation due to casting pressures, shrinkage pressures, and congealing pressures. These pressures require a ceramic body of this type to exhibit a great deal of structural integrity which generally necessitates that their transverse cross sectional profile be either circular or oval.
Such a profile is difficult to maintain over the entire length of an exhaust duct of the cylinder head because the duct must be contracted in the area of a bore for a valve introduction (German Publishing Examined Patent Application No. 2,750,290) to form a throat or depression. Such a contraction or depression must be compensated for in order to avoid a decrease in cross sectional area which would hinder the gas flow. This compensation to maintain a constant cross sectional area generally requires that the transverse cross sectional profile of the exhaust duct become kidney shaped. This kidney shaped profile is inherently weaker with respect to externally inwardly applied forces than a circular or oval profile. Tests have shown that a ceramic exhaust duct which has a contraction or depression and a resulting kidney shaped profile can not withstand the loads experienced during the casting of the cylinder head.
One object of the present invention is to provide a hollow pipe-like or tubular ceramic body which, on the one hand is able to conform to the structural requirement in a cylinder head of an engine, and on the other hand can withstand the loads imposed during the casting of that cylinder head.
According to the present invention, a tubular ceramic body is provided which has a thickened portion or external layer applied to at least the contraction or depression. This thickened portion or external layer consists of a pressure resistant material which fills in the depression and gives the ceramic body the strength needed to withstand the pressures occurring during the casting of the cylinder head.
The pressure resistant material for the thickened portion or external layer is preferably Zirconic oxide wherein the Zirconic oxide may be applied by utilizing a conventional plasma spraying means such that at least a portion of the outer surface of the ceramic body is coated. This layer exhibits a rough surface which provides good casting properties for the ceramic body and also improves its heat insulation qualitites against the cylinder head. Where the ceramic body includes a single duct which is bifurcated into two duct sections, the pressure resistant material may be applied as support between the duct sections such that the ceramic body can, without damage, withstand the pressures occurring during the casting of the cylinder head.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purpose of illustration only, an embodiment in accordance with the present invention.